The electrolytic production of alkali metal chlorate, and especially sodium chlorate, is well known. Alkali metal chlorate is an important chemical, particularly in the pulp and paper industry as a raw material for the production of chlorine dioxide that is widely used for bleaching. Conventionally, it is produced by electrolysis of alkali metal chlorides in non-divided electrolytic cells. The overall chemical reaction taking place in such cells isMCl+3H2O→MClO3+3H2 where M is an alkali metal. Examples of chlorate processes are described in inter alia U.S. Pat. No. 5,419,818 and EP 1 242 654.
During the production of sodium chlorate, sodium chloride is oxidized to form chlorine on the anode which subsequently transforms to sodium chlorate under controlled chemical conditions. On the cathode, water is reduced to form hydrogen gas as a by-product of the electrochemical reaction.
U.S. Pat. No. 3,535,216 discloses a process of producing chlorate in a chlorate cell equipped with steel cathodes.
However, steel cathodes are not stable over time in the chlorate process. Steel may also corrode in the electrolyzer. Steel cathodes may also conduct atomic hydrogen whereby connection between steel cathodes and titanium based anodes in bipolar cells may need a back-plate to prevent formation of titanium hydride. Also, it has been found that the use of sodium dichromate and molybdic acid in amounts described in U.S. Pat. No. 3,535,216 results in considerable evolution of oxygen, which is undesirable, as well as high cell voltage.
The object of the present invention is to provide a process of producing alkali metal chlorate which reduces the cell voltage. A further object is to provide a process of activating the cathode in such cell in a convenient and efficient way while using low amounts of chromium and activating metal(s). A further object of the invention is to provide a process with high cathodic current efficiency. A further object is to provide a process in which the formation of oxygen is decreased whereby energy losses and the risk of explosions in the cell also are decreased.